Class d amplifier circuit

ABSTRACT

This application relates to Class D amplifier circuits ( 200 ). A modulator ( 201 ) controls a Class D output stage ( 202 ) based on a modulator input signal (Dm) to generate an output signal (Vout) which is representative of an input signal (Din). An error block ( 205 ), which may comprise an ADC ( 207 ), generates an error signal (ε) from the output signal and the input signal. In various embodiments the extent to which the error signal (ε) contributes to the modulator input signal (Dm) is variable based on an indication of the amplitude of the input signal (Din). The error signal may be received at a first input ( 204 ) of a signal selector block ( 203 ). The input signal may be received at a second input ( 206 ) of the signal selector block ( 203 ). The signal selector block may be operable in first and second modes of operation, wherein in the first mode the modulator input signal is based at least in part on the error signal; and in the second mode the modulator input signal is based on the digital input signal and is independent of the error signal. The error signal can be used to reduce distortion at high signal levels but is not used at low signal levels and so the noise floor at low signal levels does not depend on the component of the error block ( 205 ).

RELATED APPLICATION

The present disclosure claims priority to British Patent Application No.1415328.2 filed on Aug. 29, 2014, which is incorporated by referenceherein in its entirety.

This invention relates to Class D amplifier circuits and especially to aClass D amplifier that selectively varies the signal components used todrive an amplifier output stage based on signal amplitude.

FIG. 1 illustrates an example of a conventional Class D amplifier. Amodulator 101 controls a Class D output stage 102 to generate an outputsignal Vout based on an input signal, which in this example is a digitalinput signal Din. The output stage 102 comprises a plurality of switchesfor switching the output stage 102 between a plurality of voltages, forexample a supply voltage VDD and ground, or positive and negative supplyvoltages +VDD and −VDD. The output stage 102 may comprise a half-bridgeor full-bridge switch arrangement, as will understood by one skilled inthe art, and is switched to output an analog output signal Vout, whichmay in some embodiments be a differential output signal.

The modulator 101 receives an input signal which in this example is adigital signal Din and derives at least one appropriate control signalfor the output stage 102. A standard digital modulator 101 has awell-defined, transfer function, for instance flat and with a definedgain over some pass-band range of frequency. Ideally this digital-domainsignal processing gives high performance and avoids analog circuitrywith its non-zero signal degradation associated with noise, componentmismatch and non-linearity.

However, the output signal Vout is an analog signal and its performanceis limited by analog effects in any output driver stage of theamplifier. For example, output driver transistor on-resistance, finiterise and fall times, propagation delays, power supply ripples and outputimpedance. In addition, any power supply ripple will cause aproportional gain variation of the output driver stage.

Negative feedback techniques are typically used to suppress signaldistortion arising from these causes. The Class D amplifier of FIG. 1thus has a feedback path comprising an analog-to-digital convertor (ADC)103 and subtractor 104.

The ADC 103 is adapted to receive the analog output signal Vout andproduce a digital signal representative of this output signal. Thisdigital signal is then subtracted from the digital input signal Din bythe subtractor 104 to produce an error signal. A loop filter 105, whichmay be for example a digital integrator, filters the error signal togenerate the modulator input signal which is supplied to the digitalmodulator 101.

The performance of such an amplifier circuit is limited by the noise,resolution and linearity of the ADC 102. To avoid introducing unwantednoise into the amplifier circuit, the ADC has to have good noisecharacteristics which typically requires the use of a relatively highperformance continuous-time ADC. Such an ADC is relatively large (interms of silicon area in an integrated circuit) and has a relativelyhigh power consumption in use.

Typically Class D amplifier circuits have been used in relatively highpower applications, for instance mains-powered audio apparatus wheresize and power consumption are not critical. Increasingly however, ClassD amplifiers are being considered for applications in portable devicesand the like. With a move to smaller geometry silicon fabricationprocesses it is advantageous to use circuits which are mainly digital. AClass D amplifier may comprise a largely digital architecture and thusClass D amplifiers are being increasingly proposed for use driving smallspeakers, e.g. of a portable device or the like or even forearbud/headphone applications where the powers are typically relativelylow.

In such applications size and power efficiency are factors to beconsidered for the amplifier circuit.

It is therefore desirable to provide a Class D amplifier circuit that atleast mitigates some of the above mentioned disadvantages.

According to the present invention there is provided a Class D amplifiercircuit for receiving a digital input signal and outputting an analogoutput signal comprising:

-   -   a class-D output stage;    -   a digital modulator for generating at least one control signal        for controlling said class-D output stage based on a modulator        input signal;    -   an error block for generating an error signal based on said        output signal and said digital input signal;    -   a signal selector block configured to receive the error signal        at a first input, receive a version of the digital input signal        at a second input and generate the modulator input signal;        wherein:    -   said signal selector block is operable in a first mode and a        second mode of operation, wherein:    -   in the first mode the modulator input signal is based at least        in part on the error signal; and    -   in the second mode the modulator input signal is based on the        digital input signal and is independent of the error signal; and        a signal selection controller configured to control the mode of        operation of the signal selector block based on an indication of        the amplitude of the digital input signal.

The signal selection block may comprise a first signal path between saidfirst input and a selector module and a second signal path between saidsecond input and said selector module. The selector module may beconfigured to generate the modulator input signal from the signals fromfirst signal path and the second signal path.

The selector module may be operable in: a combiner state to combine thesignal from the first signal path with the signal from the second signalpath to provide the modulator input signal, and/or a pass-through stateto provide the signal from the second signal path as the modulator inputsignal. The signal selection controller may be configured to control theselector module in the combiner state in the first mode and in thepass-through state in the second mode.

In some embodiments the selector module is configured to select eitherthe signal from the first signal path or the signal from the secondsignal path to provide the modulator input signal. The signal selectioncontroller may be configured to control the selector module to selectthe signal from the first signal path in the first mode and the signalfrom the second signal path in the second mode.

In some embodiments the second signal path comprises at least onevariable gain element. The signal selection controller may be configuredto control the at least one variable gain element to provide a firstgain setting in the first mode and a second gain setting in the secondmode. The second gain setting may be zero.

In some embodiments the signal selection controller is configured tocontrol the at least one variable gain element to provide a controlledtransition in gain between said first and second gain settings thatinvolves at least one intermediate gain setting. The controlledtransition in gain may comprise a ramp in gain.

In some embodiments the variable gain element comprises a multiplier.

The first signal path may comprise a loop filter. The at least onevariable gain element may be located in the first signal path downstreamof the loop filter. Additionally or alternatively in some embodimentsthe loop filter has a variable gain and the at least one variable gainelement may comprise said loop filter. The loop filter may comprise anintegrator and the signal selection controller may be configured tocontrol the integrator time constant of said integrator to vary the gainof the loop filter. The signal selection controller may be configured toenable the loop filter in the first mode and disable the loop filter inthe second mode.

In some embodiments the error block comprises an analog to digitalconvertor (ADC) coupled to receive the output signal. The error blockmay further comprise a subtractor which may generate the error signalbased on the output of the ADC and the digital input signal. The signalselection controller may be configured to enable the analog ADC in thefirst mode and disable the ADC in the second mode.

The amplifier circuit may further comprise an envelope detector fordetermining an envelope value for the digital input signal. The signalselection controller may be configured to use the envelope value as theindication of amplitude of the digital input signal.

The envelope detector may apply a predetermined delay between any dropin detected signal envelope and a reduction in the envelope value. Insome embodiments the envelope detector receives a version of the digitalinput signal.

In some embodiments the signal selection controller is configured toreceive an indication of a volume control setting and to use saidindication of a volume control setting as the indication of theamplitude of the digital input signal.

The signal selection controller may be configured to transition from thefirst mode of operation to the second mode of operation if theindication of the amplitude of the digital input signal drops below afirst amplitude threshold. It may also be configured to transition fromthe second mode of operation to the first mode of operation if theindication of the amplitude of the digital input signal rises above asecond amplitude threshold. The first threshold may be the same as thesecond threshold or the thresholds may be different.

The signal selection controller may be configured to initiate anytransition between the first and second modes at a time when themagnitude of the input signal is at or below a first magnitude level.The Class D amplifier circuit may comprise a low-level detector fordetecting when the magnitude of the input signal is at or below thefirst magnitude level.

Embodiments also relate to integrated circuits comprising a Class Damplifier circuit as described in any of the variants above. Furtherembodiments relate to electronic devices comprising such integratedcircuits or amplifier circuits as described in any variants above. Thedevice may be at least one of a portable device; a battery power device;a computing device; a communications device; a gaming device; a mobiletelephone; a personal media player; a laptop, tablet or notebookcomputing device.

According to another aspect of the invention there is provided a methodof operating a Class D amplifier circuit comprising: receiving a digitalinput signal; and providing a modulator input signal to a digitalmodulator so as to control a class-D output stage to produce an outputsignal; wherein the method comprises selectively varying between a firstmode of operation and a second mode of operation based on an indicationof the amplitude of the digital input signal; wherein, in the first modeof operation, the modulator input signal is based, at least partly, onan error signal, the error signal being generated based on said outputsignal and said digital input signal; and in the second mode ofoperation, the modulator input signal is based on the digital inputsignal and is independent of the error signal.

In a further aspect of the invention there is provided a Class-Damplifier circuit for receiving an input signal and outputting an outputsignal, said amplifier circuit comprising: a class-D output stage; acontroller for generating at least one control signal for controllingsaid output stage; and an error block for deriving an error signal fromsaid output signal and said input signal; wherein said controller isoperable in a first mode of operation where said at least one controlsignal is based at least partly on said error signal and also in asecond mode of operation where said at least one control signal is basedon said input signal and does not include any contribution from saiderror signal; and wherein said controller is configured to selectivelyvary between said first and second modes of operation based on anindication of the amplitude of the input signal.

In a further aspect there is provided a Class-D amplifier circuit thatchanges between open-loop and closed-loop operation based on anindication of input signal amplitude.

In a further aspect of the invention there is provided a Class-Damplifier circuit that controllably transitions between an open-loopoperational mode and a closed-loop operational mode in response to acharacteristic of the amplitude of an input signal.

In a further aspect of the invention there is provided a Class-Damplifier circuit for amplifying an input signal that selectivelytransitions between an open-loop operational mode and a closed-loopoperational mode in response to a characteristic of the amplitude ofsaid input signal.

In a further aspect of the invention there is provided a Class Damplifier circuit for receiving a digital input signal and outputting ananalog output signal comprising:

-   -   a class-D output stage;    -   a digital modulator for generating at least one control signal        for controlling said class-D output stage based on a modulator        input signal;    -   an error block for generating an error signal, said error block        comprising an analog to digital converter for receiving an        indication of said analog output signal and a subtractor for        generating said error signal based on the output of the analog        to digital converter and the digital input signal;    -   a signal selector block comprising:        -   a selector module;        -   a first signal path between a first input for receiving the            error signal and the selector module; and        -   a second signal path between a second input for receiving            the digital input signal and said selector module;    -   wherein the selector module is configured to generate the        modulator input signal from the signals from first signal path        and the second signal path; and    -   a signal selection controller configured to vary a gain applied        in said first signal path based on an indication of the        amplitude of the digital input signal.

In a further aspect of the invention there is provided a Class Damplifier circuit for receiving an amplifier input signal and outputtingan output signal comprising: a class-D output stage; a modulator forgenerating at least one control signal for controlling said class-Doutput stage based on a modulator input signal; an error block forgenerating an error signal, based on said output signal and saidamplifier input signal; a signal selector block configured to receivethe error signal at a first input and the digital input signal at asecond input and to generate the modulator input signal; and a signalselection controller configured to control the amplifier circuit to varythe extent to which the error signal contributes to the modulator inputsignal based on an indication of the amplitude of the digital inputsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a known Class D amplifier circuit;

FIG. 2 illustrates a Class D amplifier circuit according to anembodiment of the invention;

FIG. 3 illustrates a flow chart of one example of a method of swappingbetween first and second modes of operation;

FIG. 4 illustrates an example of an analog to digital convertor suitablefor use in an embodiment of the invention;

FIG. 5 illustrates an example of a current controlled oscillator;

FIG. 6 illustrates an example of a digital modulator;

FIG. 7 illustrates an example of an output driver stage suitable for usein an embodiment of the invention.

FIG. 8 illustrates an example of a loop filter suitable for use in anembodiment of the invention.

FIG. 9 illustrates a Class D amplifier circuit according to anotherembodiment of the invention;

FIG. 10 illustrates a Class D amplifier circuit according to a furtherembodiment of the invention;

FIG. 11 illustrates a Class D amplifier circuit according to a yetfurther embodiment of the invention;

FIG. 12 illustrates a Class D amplifier circuit according to anotherembodiment of the invention; and

FIG. 13 illustrates a device having a Class D amplifier circuitaccording to an embodiment of the invention.

DESCRIPTION

As mentioned previously a Class D amplifier for receiving an inputsignal and generating an output signal typically comprises a modulatorfor controlling a Class D output stage based on a modulator inputsignal. An error block, which may for instance comprise an ADC and asubtractor in a feedback path, may be provided to generate an errorsignal which is based on the output signal and the input signal, e.g.the difference between the converted version of the output signal andthe input signal. Embodiments of the present invention relate to Class Damplifier circuits in which the extent of the contribution of the errorsignal to the modulator input signal is variable based on an indicationof the amplitude of the input signal. In some embodiments the amplifiercircuit may be selectively variable between at least two modes ofoperation based on the amplitude of the input signal. In a first mode ofoperation the modulator input signal may be based at least in part onthe error signal. In a second mode of operation the modulator inputsignal may be based on the input signal but may be independent of theerror signal. At relatively high signal amplitudes the amplifier mayoperate in the first mode of operation and at relatively low signalamplitudes the amplifier may operate in the second mode, e.g. an openloop mode of operation.

Thus at high signal levels the error signal is used to help reduce anydistortion in the output signal. At low signal levels however the extentof any distortion is not so significant and thus the feedback errorsignal is not used. As the error signal is only used at relatively highsignal levels the noise requirements for the components of the errorblock, such as the ADC, are relaxed compared with a conventionalimplementation where the error signal is always used during steady stateoperation. In effect at low signal levels, where the noise floor of theamplifier circuit is more noticeable, the error signal is not used. Thismeans for example that a smaller and/or lower power ADC may beimplemented in the error block than otherwise would be required, thusreducing the size and/or increasing power efficiency of the amplifiercircuit but without any significant reduction in signal quality or noiseperformance. In some embodiments, as will be described in more detaillater, some components associated with generating or processing theerror signal may even be disabled in the second mode of operation toprovide additional power savings.

FIG. 2 illustrates a Class D amplifier circuit, generally indicated 200,according to an embodiment of the invention. A modulator 201, which inthis example is a digital modulator, controls a Class D output stage 202as described above with respect to FIG. 1. In some embodiments, theoutput driver stage 202 may be a full H-bridge or a half-bridge class Damplifier output stage or any other suitable amplifier output stage.

In the embodiment of FIG. 2 the input to the digital modulator 201 is amodulator input signal, Dm, received from a signal selector block 203.The signal selector block 203 receives, at a first input 204, an errorsignal ε from an error block 205. The signal selector block alsoreceives a version of the input signal Din at a second input 206.

The error block 205 generates the error signal ε from the digital inputsignal Din and the analog output signal Vout. In this example, the errorsignal is based on a comparison between the digital input signal Din andthe analog output signal Vout.

As illustrated in FIG. 2 the error block 205 may comprise ananalog-to-digital convertor (ADC) 207 which receives the analog outputsignal Vout and outputs a digital signal representative of the analogoutput signal. A digital subtractor 208 receives the output of the ADC207 and also a version of the digital input signal Din and generates theerror signal ε. In some embodiments, the analog output signal Vout maybe filtered by a low pass filter 209 before it is input into the ADC206. This helps to provide suppression of any high frequency signals andtransients at the output, to avoid them being mixed down to audiofrequencies by imperfections of the ADC.

The signal selector block 203 is operable to autonomously vary theextent to which the error signal ε contributes to the modulator inputsignal Dm based on an indication of the amplitude of the input signalDin. In one embodiment the signal selector block 203 is operable in twomodes of operation. In a first mode the modulator input signal Dm isbased at least partly on the error signal. The first mode thereforecorresponds to a closed loop mode of operation of the amplifier circuit.In a second mode however the modulator input signal Dm is based on thedigital input signal Din and is independent of the error signal ε. Thesecond mode thus corresponds to an open loop mode of operation of theamplifier circuit.

The signal selector block 203 thus comprises a first signal path betweenits first input 204 and a first input of a selector module 210 and asecond signal path between its second input 206 and a second input ofthe selector module 210. The selector module 210 is configured to takethe signals from one or both of the first and second signal paths toprovide the modulator input signal Dm. The signal selector module 210may be implemented in various ways as will be described in more detaillater.

The first signal path is thus a signal path for the error signal c. Thissignal path may comprise a loop filter 211 for filtering the errorsignal in a similar fashion as described above in relation to FIG. 1.The open-loop gain of the loop comprising loop filter 211 shouldpreferably be high at audio frequencies and yet also remain stable athigher frequencies. The loop filter 211 may thus for instance be a firstorder integrator, or possibly a higher order filter, with a high gain atthe audio signal band and a low gain at higher frequencies.

The second signal path is a signal path for the input signal Din. FIG. 2illustrates no signal processing in the second signal path but there maybe one or more signal conditioning components such as a filter locatedin this signal path if required.

As mentioned above the signal selection block 203 is operable in a firstmode where the modulator input signal is based, at least partly on theerror signal. Thus in the first mode of operation there is a errorsignal contribution from the first signal path to the modulator inputsignal Dm. In the second mode of operation the modulator input signal isbased on the input signal Din and is independent of the error signal,thus in the second mode of operation there is an input signalcontribution from the second signal path to the modulator input signalDm and effectively no signal contribution from the first signal path (atleast no signal contribution that corresponds to the error signal).

There are various ways in which the first and second modes of operationmay be enabled and, as mentioned above, the selector module 210 may takevarious forms. For example in some embodiments the selector module mayreceive signal contributions from both the first and second signal pathsand may select either the signal from the first signal path or thesecond from the second signal path to provide the modulator input signalDm. The selector module 210 may therefore comprise a switch moduleswitching its output between the two signal paths. The selector module210 may be controlled by a signal selection controller 212. The switchmodule could comprise one or more physical switch elements and/or may beimplemented by a digital switch which may be a physical multiplexercomprising combinatorial logic elements or may, for instance, change theaddress of a register from which the signal data is acquired from. Insuch embodiments therefore the signal from the first signal path, e.g.the filtered error signal, is used in the first mode (without acontribution from the second signal path) and the signal from the secondsignal path, e.g. the input signal Din, is used in the second modewithout any contribution from the first signal path. In such anembodiment however there could be a gross transient in switching betweenmodes, for instance as the loop filter settles out following a change tothe first mode or as an error component is removed when switching to thesecond mode.

In some embodiments therefore the selector module may 210 may be acombiner for, at least in the first mode, combining the signals from thefirst and second signal paths to provide the modulator input signal Dm.In the first mode the selector module 210 may therefore be a combinerwhich functions as an adder. In such an embodiment the filtered errorsignal from the first signal path may be added to the input signal fromthe second signal path in the first mode. In the second mode the signalfrom the second signal path may be the only contribution to themodulator input signal Dm.

The combiner selector module 210 may therefore be operable in twostates: State 1 which is a combiner state of operation to combine thesignals from the first and second signal paths in the first mode andState 2 which is a pass-through state of operation in which only thesignal from the second signal path is used. The relevant state ofoperation of the selector module 210 may be controlled by the controller212. Additionally or alternatively the amplifier circuit could beconfigured so that no signal dependent contribution of the error signalis received at the selector module 210 in the second mode. In otherwords in the second mode the signal from the second signal path receivedat the combiner selector module 210 has a constant, non signaldependent, quiescent value, e.g. zero. In which case the selector module210 may comprise a simple adder or the like. The adder may operate inthe same way in the first and second modes but in the second mode thereis no signal component received at the adder from the error signal andthus the modulator input signal Dm will be independent of the errorsignal. In some embodiments however it may be preferred for thecontroller 212 to operate the combiner selector module 210 in apassthrough state in the second mode of operation, even when there is nosignal dependent contribution received from the second signal path, toavoid the computational expense of repeatedly adding zero to the signalfrom the first signal path.

There are various ways in which the contribution of the error signal tothe input of the selector module 210 may be removed. For instance, thesignal in the first signal path may be effectively blocked or attenuatedto zero in the second mode. In some embodiments there may be at leastone variable gain element 213 located in the first signal path thatapplies a controlled gain. The signal selection controller 212 may beconfigured to control the at least one variable gain element 213 toprovide a first gain setting in the first mode, which is a nominalnon-zero gain setting. In the second mode the gain may be set to zero.Additionally or alternatively the loop filter 211 may have a variablegain which can be set to zero by the controller 212 in the second modeof operation.

In some embodiments no error signal may be generated in the second modeof operation. For instance the one or more components of the error block205 could be controlled such that the output of the error block is aconstant zero so that there is no error signal received at the firstinput of the signal selector block 203 in the second mode of operation,and thus no error signal dependent contribution received at the selectormodule.

It will therefore be clear that in some embodiments when the signalselection block 203 is operating in the first mode the (signaldependent) error signal is added to the input signal from the secondsignal path to form the modulator input signal Dm. However in the secondmode only the input signal from the second signal path is used for themodulator input signal. In such embodiments any gross transient involvedin switching between modes is likely to be lower than that discussedabove in relation to embodiments that simply step change between usingonly the input signal or only the error signal.

In some embodiments, to minimise or eliminate any unwanted transients,there may be a controlled transition in gain applied in the first signalpath during a mode change between the first and second modes. Forexample as mentioned there may be a variable gain element in the firstsignal path, such a digital multiplier 213. FIG. 2 illustrates thevariable gain element 213 located downstream of the loop filter 211 butequally a gain element could additionally or alternatively be locatedupstream of the loop filter 211 and/or, as mentioned, the gain of theloop filter 211 may itself be variable. The signal selection controller212 may be configured to control the at least one variable gain element,e.g. multiplier 213, to provide said a first, non-zero gain setting inthe first mode and a second gain setting in the second mode. The secondgain setting may be zero but in some embodiments may be a non-zero gainsetting which is lower than the first gain setting. The variable gainelement may be controlled to provide a controlled transition in gainbetween the first and second gain settings that involves at least oneintermediate gain setting. For instance there may be a ramp in gain overa certain time period. For example consider that the signal selectorblock changes mode from the first mode of operation to the second modeof operation. In the first mode of operation the gain applied to thefirst signal path is at the first gain setting. The controller 212 maythen initiate a controlled transition in gain during the mode change tothe second mode. The gain applied by multiplier 213 may then becontrollably ramped down over a defined period of time to the secondgain setting. The second gain setting may be zero in which case thesecond mode is enabled when the gain reaches zero. In other embodimentshowever the second gain setting may be a small but not zero gain settingwhich is low enough that there will be no significant transients if theselector module 210 is switched to the pass-through state in the secondmode of operation.

The signal selection controller 212 determines whether to operate in thefirst or second modes of operation based on an indication of theamplitude of the input signal Din. In some embodiments, as illustratedin FIG. 2, an envelope detector 214 may be provided to derive anenvelope value ENV for the digital input signal, which is provided tothe controller 212. In some embodiments the envelope detector 214 mayreceive the input digital signal Din. In some embodiments however asignal from a signal path from the amplifier circuit could be used todetermine an envelope value instead, for instance a signal derived fromthe modulator 201. It should be appreciated however that alternativelyan indirect indication of the amplitude of the digital input signal Dincould be provided, for example a volume control signal Vol could betaken as an indication of a maximum amplitude. In other arrangements asignal such as an envelope value may be determined upstream and suppliedto the amplifier circuit, or the downstream loading characteristicscould be used as an indication of signal amplitude.

In the embodiment of FIG. 2 the controller 212 receives the envelopevalue ENV and determines whether to operate in the first mode ofoperation or the second mode of operation. If ENV indicates the inputsignal has a relatively large amplitude the first mode of operation isselected, i.e. a closed loop mode of operation for the amplifier. If ENVindicates that the input signal has a relatively small amplitude thenthe second mode of operation is selected i.e. in this embodiment, anopen loop mode of operation. In some embodiments the signal selectioncontroller 212 may be configured to transition from the first mode ofoperation to the second mode of operation if the envelope value, i.e.the indication of the amplitude of the digital input signal Din, dropsbelow a first amplitude threshold, and to transition from the secondmode of operation to the first mode of operation if the envelope valuerises above a second amplitude threshold. The first and second amplitudethreshold may be the same or may be different, for instance to applysome hysteresis.

Therefore, when the signal is relatively low the error signal is notused, as the low amplitude input signal results in an output signalwhich is less affected by any intrinsic distortion in the output driverstage, e.g. caused by any potential power supply ripples or othersources of error. The noise floor of the amplifier system is thus mainlydetermined by the digital modulator 201 which can be designedaccordingly. When the amplitude of the digital input signal Din isrelatively high however, the output signal will suffer more intrinsicdistortions. Therefore, at higher amplitudes the amplifier operates inthe closed loop mode so that the error caused by these distortions canbe reduced. The components of the error block such as the ADC 207 arethus designed to reduce distortion etc. at higher signal levels. Athigher signal levels the noise floor performance of the ADC 207 is lessimportant however, and thus the ADC 207 can be designed with relaxedconstraints compared to a conventional Class D amplifier with such afeedback loop that always operates in steady state operation. This dualmode, i.e. open/closed loop, operation of the Class D amplifieradvantageously allows the use of a smaller ADC and/or an ADC thatconsumes less power than would otherwise be the case. By being able toswitch between the closed loop and open loop modes of operation thesystem as a whole enables the power consumption of the ADC to beoptimised whilst still maintaining a high performance response.Furthermore, being able to transition in a controlled manner between theclosed loop and open loop modes of operation advantageously minimisesany output signal artefacts.

FIG. 3 illustrates a flow chart of one example of how the amplifierillustrated in FIG. 2 may transition between modes according to anembodiment of the invention.

If the signal peak level reduces 301 a, the envelope detector value ENVmay decrease 302 a in accordance with a decay time constant, andpossibly a hold time of the envelope detector. To avoid changing betweenmodes unnecessarily often the envelope detector 214 may apply a holdtime before reducing the envelope value in response to a reduction inpeak signal level and may apply a relatively slow decay constant. If thesignal peak level increases 301 b, the envelope detector value ENV mayincrease 302 b in accordance with an attack time constant. If theenvelope detector block 214 employs a relatively short attack time, thiswill ensure that rapid spikes in the digital input signal Din willresult in a rapid reaction by the envelope detection circuitry 214 andthus a rapid response can be made so as to transfer to a closed loopmode of operation, and ensure that the feedback loop is implemented toreduce any error caused by the higher signal in the output driver stage.In contrast, the long decay time will avoid unnecessary switching of theoperation modes as it is quite likely that one high-amplitude signalpeak will be followed in quick succession by another. In someembodiments pre-emphasis filtering could also be used to exaggerate andanticipate the rising edges of the signal.

The signal selection controller 212 monitors 303 the signal indicativeof the amplitude of the input signal, e.g. the envelope value. Thecontroller 212 may for example compare the signal ENV to at least onepredetermined threshold value to determine automatically whether toenter the open loop mode (second mode) or closed loop mode (first mode).There may be a single amplitude threshold or there may be differentamplitude thresholds: one threshold for changing from the first to thesecond mode; and another, different threshold for changing from thesecond to the first mode so as to apply hysteresis to the mode changes.

If the amplitude of the signal relative to the relevant predeterminedthreshold is unchanged then the mode of operation is unchanged and thesignal selection controller continues to monitor 303 the indicativesignal ENV.

If the indicative signal ENV falls below the relevant predeterminedthreshold, then the signal selection controller 212 will decide to enterinto open loop mode (i.e. the second mode) as in step 304. In thisexample the gain applied by a variable gain element in the first signalpath, such as multiplier 213, is ramped down from the first gain settingto zero in step 305. A predetermined gain ramp will be applied over arelatively short period of time but a sufficient period of time not tocause significant transients in the output signal. Once the gain hasreached zero, the ADC 207 and/or the loop filter 211 may be disabled instep 306. Disabling the ADC and/or loop filter when not required helpssave power and increase the efficiency of the amplifier circuit. Theselector module 210 may also be put into pass-through mode in step 307so as to just use the input signal from the second signal path. Byallowing the gain applied by the variable gain element 212 to ramp tozero before disabling the ADC or putting the selector module intopass-through mode, reduces any gross transients and thus output signalartefacts which would be caused by just suddenly disabling the ADC orswitching, i.e. step-changing, the selector module into pass-throughmode. It should be noted that both of steps 306 and 307 are optional andone or both steps may not be used in some embodiments. Also the order ofsteps 306 and 307 could be reversed. In addition the step 306 ofdisabling a component such as the loop filter may in some embodimentsonly be implemented if the signal amplitude remains low for a certaintime period and/or if the signal amplitude drops below another lowerthreshold. It may take time and/or power to re-enable the loop filterand thus it may be disabled only in instance when it is likely to bedisabled for more than a very short time.

Referring back to step 303, if the indicative signal ENV goes above therelevant predetermined threshold the signal selection controller 212will decide to enter into the closed loop mode (first mode) as in step308. In this example the ADC 207 and/or loop filter 211 is first enabledin step 309. This may involve waiting a short period of time to allowthe ADC/loop filter to settle. As the amplifier circuit is still inopen-loop mode at this stage, the gain applied by the variable gainelement, e.g. multiplier 213 will already be set at zero—although if theloop filter 211 is the variable gain element and has been disabled itshould be re-enabled with a gain of zero. If the filter 211 has beenoperational it may have a non-zero output which is likely to beincorrect and thus the output of the loop filter 211 may be reset tozero. In step 311 the selector module is put into the combiner statewhere it combines the signals from the first and second signal paths.However, as the gain applied to the first signal path is still zero, atthis stage there is no contribution from the first signal path and thusno transient. In step 312, the gain applied by the variable gainelement, e.g. multiplier 213 is then ramped up to the nominal first gainsetting for the first signal path, the ramp being applied over asuitable time frame to avoid any significant transients. Similar to thatdescribed above, this control by the signal selection controller 212helps to reduce any gross transients and thus signal artefacts in thesignal which may otherwise be audible.

To further reduce any transients occurring on transition between modesthe signal selector controller 212 may be configured to only initiate atransition between modes when the instantaneous value of the inputsignal is at or below a low magnitude level or threshold, e.g. at ornear zero. The amplifier circuit may comprise a low-level detector suchas a zero crossing detector to detect a low instantaneous signalmagnitude and the signal selector controller may wait for a zerocrossing to initiate a transition between modes.

In open-loop mode, the amplifier gain from Din to Vout is defined by thegain of the digital modulator and output driver and the gains of anyother elements included in the signal path from Din to Vout. Inclosed-loop mode, the gain from Din to Vout is defined by the inverse ofthe conversion gain of the ADC and similar allowances for any gainscaling in the signal paths from Vout and Din to the subtractor 208,independently of the value of gain of the variable gain element in theloop filter path provided the open-loop loop gain is adequately high.These open-loop and closed-loop amplifier gains may be designed,calibrated or continuously adjusted in use to be nominally equal.However any signal artefacts due to mismatch in the amplifier gain fromDin to Vout between first and second modes will also be reduced by thegradual controlled transition or ramp of the variable gain.

As mentioned above the embodiments of the invention can be implementedwith ADCs that consume less space and/or power than the ADCs used inconventional amplifier circuits without a corresponding decrease inamplifier noise performance.

FIG. 4 illustrates an example of an ADC 207 according to an embodimentof the invention. In this example the ADC 207 comprises avoltage-to-current conversion block 401 and a current controlledoscillator 402. The number of pulses of the output of thecurrent-controlled oscillator 402 is counted by a counter 403. Thecounter output is outputted, via calibration block 404, as the ADCoutput signal. The transfer function of this current-controlled circuitis more immune to process and temperature than a voltage-controlledcircuit.

As shown in FIG. 5, the current-controlled oscillator 402 may beimplemented by a simple ring oscillator of CMOS inverters as will beappreciated by one skilled in the art.

The conversion gain of the current-controlled oscillator 402 is processdependent and may be adjusted by calibration on start-up under thecontrol of a calibration engine 405. This will generate an overall gaincorrection factor p and/or a polynomial correction via the coefficientsq_(i) which can be applied by the calibration block 404.

The ADC 207 noise characteristics may thus dominate the overallamplifier noise performance in closed loop mode, but as closed loop modeis only used at high signal amplitudes the noise floor is not sosignificant. For low signal levels open loop mode is used and thedigital modulator performance thus defines system performance in termsof noise.

The digital modulator 201 could, for example as illustrated in FIG. 6,be a simple digital ramp modulator. The modulator input signal Dm may belatched, i.e. temporarily stored, in a storage element such as a clockedregister 601 that is controlled by a clock signal CK. The output of thestorage element 601 is compared by a digital comparator 602 with theoutput signal of a counter 603. The counter 603 is supplied with theclock signal CK supplied to the register 601 and a relatively fasterclock signal CKF and produces a digital ramp signal as will beunderstood by those skilled in the art. The digital ramp signal outputfrom the counter 603 will be a PWM-type signal with a cycle perioddefined by the relatively slower clock signal CK and with a pulse timingresolution defined by the fast clock CKF. It will of course beappreciated that a closed-loop ramp modulator, or a hysteretic,self-oscillating modulator could be implemented if desired.

As mentioned above the output stage may be any suitable Class D outputstage. FIG. 7 illustrates one example of an output driver stage 202 thatcould be implemented in an embodiment of the invention. In thisembodiment the output driver stage 202 comprises a pre-driver and aClass-D three-level half-bridge. It will be appreciated that a 2-levelhalf bridge or a 2 or 3-level full bridge could be implemented instead.

The use of the 3-level half bridge has the advantage that for smallsignals the output is at ground for the majority of the time. Thus theavailability of the ground signal may reduce the EMI and switching powerconsumption and may improve the power supply rejection of the outputdriver stage 2032

FIG. 8 illustrates an example of a loop filter 209 according to anembodiment of the invention. this is an example of a digital integratorloop filter where the integrator time constant may be changed by varyingα. This effectively changes the gain applied by the loop filter to theerror signal ε. It will be appreciated that a second order loop filtercould also be used.

FIG. 9 illustrates a Class D amplifier circuit according to anembodiment of the invention. In this figure the components which aresimilar to those described with reference to FIG. 2 have been given thesame references.

As can be seen multiple signal conditioning blocks 901 may beimplemented in the various signal paths and at various different placeswithin the amplifier circuit. These signal conditioning blocks 901 maycomprise various signal conditioning circuits and perform various signalconditioning functions as will be readily appreciated and understood byone skilled in the art. A non-exhaustive list of examples of such signalconditioning blocks that may be used in various configurations andcombinations include: filter blocks (High Pass/Low Pass/Band Pass etc.);gain blocks; attenuation blocks; up-samplers; down-samplers;interpolators; word-length reduction/increase blocks etc. Furthermore,in this embodiment the positions of the variable gain element 213 andthe loop filter 211 have been optionally reversed.

Other intermediate operations could also be included. For example, if aspeaker is acting as a load receiving the analog output signal Vout, theClass D amplifier circuit may include a speaker voice coil excursionlimiting function. The impedance of the coil may be extracted based onthe voice coil current and voltage and from this the excursion of thevoice coil can be estimated. Alternatively, the voice coil excursion maybe predicted from the input digital signal and a predefined orcalibrated electro-mechanical model of the voice coil. If the excursionof the voice coil indicates that there is a risk of causing mechanicaldamage or overheating of the speaker, the digital input signal may beattenuated.

Furthermore, in addition to the normal control mechanisms describedabove with reference to FIGS. 2 and 3, there may be other conditions inwhich the open loop mode of operation is desirable. For example thesemay include fault conditions in which the output appears to beunexpectedly overloaded or the loop appears to be oscillating orlocking-up, perhaps due to a gross EMI event. Thus, the envelopedetector 214 may also receive signals from the digital modulator 201,the loop filter 211 or the output node via the ADC indicating that thereis some clipping at a particular node or another fault condition hasoccurred.

FIG. 10 illustrates a Class D amplifier circuit according to anembodiment of the invention. In this figure the components which aresimilar to those described with reference to FIG. 2 have been given thesame references.

In this embodiment the error block 205 comprises a digital to analogconvertor (DAC) 1001 which receives the digital input signal and outputsan analog signal representative of the digital input signal which isinput into analog subtractor 1002 along with the output signal Vout. TheADC 207 has been repositioned to receive the signal from the output ofthe analog subtractor 1002. This has the advantage that, as the ADC nowonly has to receive the analog error signal, which is likely to be arelatively small signal, the requirements of the ADC can be relaxed evenmore than in the embodiment described in reference to FIG. 2.

FIG. 11 illustrates a Class D amplifier circuit according to anembodiment of the invention. In this figure the components which aresimilar to those described with reference to FIG. 2 have been given thesame references.

In this embodiment, the input signal is an analog input signal Ain.

The analog input signal Ain may be supplied to a main signal path 1101which outputs a digital input signal Din, representative of the analoginput signal Ain. This digital input signal is supplied to the signalselector block 203 as the digital input signal.

In this example, the error block 205 receives the analog input signalAin instead of the digital input signal Din. Ain is input into theanalog subtractor 1002 along with the analog output signal Vout.

Similar to the embodiment described in reference to FIG. 10, the ADC 206has been repositioned to receive the error signal from the output of theanalog subtractor 1002.

The envelope detector 114 may be analog envelope detector which receivesthe analog input signal Ain or it may receive the digital version Dingenerated by ADC 1101.

FIG. 12 illustrates a Class D amplifier according to an embodiment ofthe invention. In this figure the components which are similar to thosedescribed with reference to FIG. 2 have been given the same references.

In this embodiment the error block only comprises an analog subtractor1002 and receives an analog input signal Ain and the analog outputsignal Vout. The error signal generated is therefore analog and thesignal selector block 203 receives the analog input signal and theanalog error signal. An ADC 1201 is then positioned to receive theoutput of the signal selector block, which functions similar to asdescribed in reference to FIG. 2 except implementing analog components,and outputs a digital signal for inputting into the digital modulator201.

As mentioned above Class D amplifier circuits according to embodimentsof the present invention are particular suitable for use in portabledevices such a mobile telephone or laptop, notebook or tablet computingdevices of the like.

FIG. 13 illustrates a device 1300 comprising a Class D amplifier circuitaccording to an embodiment of the invention. The device may havewireless communication capability and may for example receivetransmissions via an antenna 1301. The device has at least one speaker1302. It will be appreciated that there may be more than one antenna andmore than one speaker present. An audio hub 1303 receives audio signalsfrom a number of possible sources. For instance the audio hub 1303, i.e.an audio codec, may receive signals from a modem 1304 which may encodereceived signals from the antenna into a digital format. The audio hub1303 may pass such received audio signals on to a Class D amplifier 1305according to the present invention which outputs an analog signal to beinput into the speaker 1302. Additionally or alternatively audio datamay be received at the audio hub 1303 from an audio data memory 1306.Additionally or alternatively the Class D amplifier may in some instancebe supplied with signal data, coefficient data or software code storedin a program/data memory 1307. This memory may for example be EEPROM orstatic RAM and to store the data or code in a persistent ornon-transitory form. The device may contain a power source 1308 and aform of user interface 1309 which may be a keypad, a touch screen, or anexternal controller. The device may be a portable device such as an mp3player or portable computer, or a portable communications device forexample a cellphone or tablet.

As used herein the term block refers to a functional unit or modulewhich may be implemented by one or more circuit components and which mayfor instance comprise dedicated circuitry. A block may additionally oralternatively comprise one or more software modules running, forinstance on a general purpose processor or suitably programmed FGPAarray or the like. The components of a block do not need to bephysically co-located and the components of one block may in someapplications be shared with components of another block.

In terms of the signal selection block it will be appreciated that theinputs to the signal selection block do not have to be a definedterminal of the amplifier circuit and an input to the relevant blockcould simply be any node along a relevant signal path leading to thecomponents forming the block.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in a claim,“a” or “an” does not exclude a plurality, and a single feature or otherunit may fulfil the functions of several units recited in the claims.Additionally the term “gain” does not exclude “attenuation” andvice-versa. Any reference numerals or labels in the claims shall not beconstrued so as to limit their scope.

1. A Class D amplifier circuit for receiving a digital input signal andoutputting an analog output signal comprising: a class-D output stage; adigital modulator for generating at least one control signal forcontrolling said class-D output stage based on a modulator input signal;an error block for generating an error signal based on said outputsignal and said digital input signal; a signal selector block configuredto receive the error signal at a first input, receive a version of thedigital input signal at a second input and generate the modulator inputsignal; wherein: said signal selector block is operable in a first modeand a second mode of operation, wherein: in the first mode the modulatorinput signal is based at least in part on the error signal; and in thesecond mode the modulator input signal is based on the digital inputsignal and is independent of the error signal; and a signal selectioncontroller configured to control the mode of operation of the signalselector block based on an indication of the amplitude of the digitalinput signal.
 2. A Class D amplifier circuit as claimed in claim 1wherein said signal selection block comprises a first signal pathbetween said first input and a selector module and a second signal pathbetween said second input and said selector module, wherein the selectormodule is configured to generate the modulator input signal from thesignals from first signal path and the second signal path.
 3. A Class Damplifier circuit as claimed in claim 2 wherein said selector module isoperable in a combiner state to combine the signal from the first signalpath with the signal from the second signal path to provide themodulator input signal and operable in a pass-through state to providethe signal from the second signal path as the modulator input signal andwherein signal selection controller is configured to control theselector module in the combiner state in the first mode and in thepass-through state in the second mode.
 4. A Class D amplifier circuit asclaimed in claim 2, wherein said selector module is configured to selecteither the signal from the first signal path or the signal from thesecond signal path to provide the modulator input signal and whereinsaid signal selection controller is configured to control the selectormodule to select the signal from the first signal path in the first modeand the signal from the second signal path in the second mode.
 5. AClass D amplifier circuit as claimed in claim 2 wherein said firstsignal path comprises at least one variable gain element and whereinsaid signal selection controller is configured to control the at leastone variable gain element to provide a first gain setting in the firstmode and a second gain setting in the second mode.
 6. A Class Damplifier circuit as claimed in claim 5 wherein said second gain settingis zero.
 7. A Class D amplifier circuit as claimed in claim 5 whereinsaid signal selection controller is configured to control the at leastone variable gain element to provide a controlled transition in gainbetween said first and second gain settings that involves at least oneintermediate gain setting.
 8. A Class D amplifier circuit as claimed inclaim 5 wherein said at least one variable gain element comprises a loopfilter having a variable gain.
 9. A Class D amplifier circuit as claimedin claim 8 wherein the loop filter comprises an integrator and thesignal selection controller is configured to control the integrator timeconstant of said integrator to vary the gain of the loop filter.
 10. AClass D amplifier circuit as claimed in claim 2 wherein said firstsignal path comprises a loop filter and said signal selection controlleris configured to enable the loop filter in the first mode and disablethe loop filter in the second mode.
 11. A Class D amplifier circuit asclaimed in claim 1 wherein the error block comprises an analog todigital convertor coupled to receive the output signal and wherein saidsignal selection controller is configured to enable the analog todigital converter in the first mode and disable the analog to digitalconverter in the second mode.
 12. A Class D amplifier circuit as claimedin claim 1 further comprising an envelope detector for determining anenvelope value for the digital input signal wherein signal selectioncontroller is configured to use said envelope value as the indication ofamplitude of the digital input signal.
 13. A Class D amplifier circuitas claimed in claim 1 wherein said signal selection controller isconfigured to receive an indication of a volume control setting and touse said indication of a volume control setting as the indication of theamplitude of the digital input signal.
 14. A Class D amplifier circuitas claimed in claim 1 wherein the signal selection controller isconfigured to transition from the first mode of operation to the secondmode of operation if the indication of the amplitude of the digitalinput signal drops below a first amplitude threshold and to transitionfrom the second mode of operation to the first mode of operation if theindication of the amplitude of the digital input signal rises above asecond amplitude threshold.
 15. A Class D amplifier circuit as claimedin claim 1 wherein the signal selection controller is configured toinitiate any transition between the first and second modes at a timewhen the magnitude of the input signal is at or below a first magnitudelevel.
 16. A Class D amplifier circuit as claimed in claim 15 comprisinga low-level detector for detecting when the magnitude of the inputsignal is at or below the first magnitude level.
 17. An electronicapparatus comprising a Class D amplifier circuit as claimed in claim 1.18. An electronic apparatus as claimed in claim 17 wherein saidapparatus is at least one of: a portable device; a battery power device;a computing device; a communications device; a gaming device; a mobiletelephone; a personal media player; a laptop, tablet or notebookcomputing device.
 19. A Class-D amplifier circuit that changes betweenopen-loop and closed-loop operation based on an indication of inputsignal amplitude.
 20. A Class-D amplifier circuit that controllablytransitions between an open-loop operational mode and a closed-loopoperational mode in response to a characteristic of the amplitude of aninput signal.
 21. A Class-D amplifier circuit for amplifying an inputsignal that selectively transitions between an open-loop operationalmode and a closed-loop operational mode in response to a characteristicof the amplitude of said input signal.